This invention relates to a novel composition useful for preparing a semi-permeable cellulose ester membrane useful for liquid separations. This invention further relates to a process utilizing said composition to prepare a semi-permeable cellulose ester membrane useful for liquid separations. This invention still further relates to a semi-permeable cellulose ester membrane containing a certain organic compound or compounds useful for liquid separations.
Cellulose ester membranes have long been used in membrane liquid separation processes such as microfiltration, ultrafiltration, dialysis, and reverse osmosis. Typically, cellulose ester membranes are prepared by extruding, molding, or casting the membranes from blends containing a polymer, a solvent, and an optional non-solvent. Solvents are compounds in which the polymer substantially dissolves at the membrane fabrication temperature. Non-solvents are compounds in which the polymer is substantially insoluble at the membrane fabrication temperature. Solvents which have been used to prepare cellulose ester membranes include sulfolane, dimethylformamide, N-methylpyrrolidone, and acetone. Non-solvents which have been used for cellulose esters include methanol, propanol, water, and maleic acid. Polyethylene glycol has been used as a non-solvent for cellulose triacetate. Even residual amounts of such solvents and non-solvents generally cannot be left in the membranes because they cause unacceptable contamination of the fluids being treated. Avoiding such contamination is particularly important in the treatment of blood by dialysis or the desalination of drinking water by reverse osmosis. The solvents and non-solvents are therefore typically completely removed during membrane fabrication by extensive leaching. Once the solvents and non-solvents are removed from the membranes, they present problems of disposal or extensive repurification before reuse.
Following formation of the membranes and removal of the solvents and non-solvents, it is often desirable to dry the water-wet cellulose ester membranes prior to fabrication of devices, storage, or shipment. However, cellulose ester membranes generally should not be directly dried without pretreatment because direct drying may cause adverse structural changes such as crazing or pore collapse which adversely affect membrane performance. Therefore, the pore structure of the membranes preferably is protected during the drying process. This is generally accomplished by incorporating a non-volatile, water-soluble compound such as glycerol into the pore structure of the membranes prior to drying. The non-volatile, water-soluble material also preferably serves as a surfactant or wetting agent for the later rewetting of the membranes. Such a process is commonly called "replasticization."
Such membrane preparation processes are complex, time consuming, and expensive because they require complete removal of the extruding, molding, or casting solvents and non-solvents, followed by replasticization with a non-volatile compound if the membranes are to be dried. The extrusion, molding, or casting solvents must be able to dissolve the cellulose ester to form an extrudable, moldable, or castable homogeneous blend. However, if such a compound is also used for the replasticization agent, membrane integrity could be adversely affected. Therefore, to date, the solvents used for extrusion, molding, or casting necessarily differ from the replasticization agents, resulting in a multi-step membrane preparation process. Such complex, multi-step membrane fabrication processes may result in significant variations in membrane performance.
What is needed are extrusion, molding, or casting compositions for cellulose esters in which the solvents and non-solvents are not harmful or deleterious or which can be converted into substances which are not harmful or deleterious in the end use of the membranes. Accordingly, such solvents and non-solvents would not have to be leached from the membranes prior to use. Furthermore, solvents and non-solvents which function both as plasticizers during extrusion, molding, or casting and replasticization agents during drying are highly desired.